Evidence
This page contains a snapshot of featured content which highlights evidence addressing key clinical questions including areas of uncertainty. Please see the main topic reference list for details of all sources underpinning this topic.
BMJ Best Practice evidence tables
Evidence tables provide easily navigated layers of evidence in the context of specific clinical questions, using GRADE and a BMJ Best Practice Effectiveness rating. Follow the links at the bottom of the table, which go to the related evidence score in the main topic text, providing additional context for the clinical question. Find out more about our evidence tables.
This table is a summary of the analysis reported in a guideline (underpinned by a systematic review) that focuses on the above important clinical question.
Confidence in the evidence is very low or low where GRADE has been performed and the intervention may be more effective/beneficial than the comparison for key outcomes. However, this is uncertain and new evidence could change this in the future.
Population: Pregnant women with gestational diabetes ᵃ
Intervention: Tighter glucose control
Comparison: Less tight glucose control
| Outcome | Effectiveness (BMJ rating)? | Confidence in evidence (GRADE)? |
|---|---|---|
Fasting blood glucose <5.3 mmol/L versus ≥5.3 mmol/L in women with gestational diabetes | ||
Pre-eclampsia | Favours intervention | Very Low |
Large for gestational age | Favours intervention | Very Low |
Strict control of 1.5 hour postprandial blood glucose (< 6.7mmol/L) versus customary control (<7.8 mmol/L) in women with pre-existing type 1 diabetes ᵃ | ||
Mean HbA1c (by trimester) ᵇ | No statistically significant difference | Very Low |
1 to 2 hour postprandial blood glucose of ≤7.8 mmol/L versus >7.8 mmol/L in women with pre-existing diabetes ᵃ | ||
Macrosomia at 29 to 32 weeks’ gestation | Favours intervention | Very Low |
2 hour postprandial blood glucose <6.4 mmol/L versus ≥6.4 mmol/L in women with gestational diabetes | ||
Pre-eclampsia | Favours intervention | Very Low |
Large for gestational age | Favours intervention | Very Low |
Recommendations as stated in the source guideline The National Institute of Health and Care Excellence (NICE) guideline on Diabetes in Pregnancy makes the following recommendation: Advise pregnant women with any form of diabetes to maintain their capillary plasma glucose below the following target levels, if these are achievable without causing problematic hypoglycaemia: Fasting: 5.3 mmol/L (95 mg/dL) and 1 hour after meals: 7.8 mmol/L (140 mg/dL) or 2 hours after meals: 6.4 mmol/L (115 mg/dL).
Note The guideline committee noted that some of the included studies used very short gestational intervals and that blood glucose control may require adjusting for women depending on their personal circumstances and treatment. ᵃ The guideline committee considered evidence for pregnant women with type 1 diabetes, type 2 diabetes, or gestational diabetes. They extrapolated the evidence to all women with diabetes during pregnancy, as ideally blood glucose levels during pregnancy should be as near to normal as is possible without increasing the risk of hypoglycaemia due to the linear relationship between maternal blood glucose and the risk of complications, such as macrosomia. This table therefore reports the evidence in women with gestational diabetes and any relevant indirect evidence from pregnant women with pre-existing diabetes, which is included in the guideline recommendation, when no direct evidence was available. ᵇ The guideline committee included data for the first, second, and third trimesters, all of which show no statistically significant difference between treatment groups, underpinned by very low-quality evidence.
This evidence table is related to the following section/s:
This table is a summary of the analysis reported in a guideline (underpinned by a systematic review) that focuses on the above important clinical question.
Confidence in the evidence is very low or low where GRADE has been performed and the intervention may be more effective/beneficial than the comparison for key outcomes. However, this is uncertain and new evidence could change this in the future.
Population: Pregnant women with gestational diabetes, but who are presumed to not have pre-existing diabetes
Intervention: Diet strategy/advice (with or without insulin use) and/or exercise regimen (with or without diet strategy/advice)
Comparison: Standard care or each other
| Outcome | Effectiveness (BMJ rating)? | Confidence in evidence (GRADE)? |
|---|---|---|
Dietary strategy/advice versus standard care | ||
Caesarean section | No statistically significant difference | Very Low |
Vaginal delivery | No statistically significant difference | Low |
Induction of labour | No statistically significant difference | Very Low |
Treatment failure (requirement for insulin) | Favours comparison ᵃ | Moderate to High ᵇ |
Large for gestational age | Favours intervention | Very Low |
Shoulder dystocia | Favours intervention | Very Low |
Diet plus insulin versus diet alone | ||
Caesarean section | No statistically significant difference | Very Low to Low ᵇ |
Treatment failure (requirement for insulin or increased dose of insulin) | No statistically significant difference | Low |
Large for gestational age | No statistically significant difference | Low |
Shoulder dystocia | See note ᶜ | Very Low/Moderate ᵇ |
Exercise versus no exercise | ||
Caesarean section | No statistically significant difference | Very Low |
Treatment failure (requirement for insulin) | See note ᵈ | Very Low |
Macrosomia (>4000g) | No statistically significant difference | Very Low |
Diet plus exercise versus diet alone | ||
Treatment failure (requirement for insulin) | No statistically significant difference | Very Low |
Recommendations as stated in the source guideline The National Institute of Health and Care Excellence (NICE) guideline on Diabetes in Pregnancy makes the following recommendations: Offer women advice about changes in diet and exercise at the time of diagnosis of gestational diabetes. Advise women with gestational diabetes to eat a healthy diet during pregnancy, and emphasise that foods with a low glycaemic index should replace those with a high glycaemic index. Refer all women with gestational diabetes to a dietitian. Advise women with gestational diabetes to take regular exercise (such as walking for 30 minutes after a meal) to improve blood glucose control.
Note The guideline committee stated that the critical outcomes were shoulder dystocia and large for gestational age. Mode of delivery and treatment failure were also considered important for decision making. See the full guideline for any additional outcomes reported by the included studies. The guideline committee were surprised that the addition of insulin did not have a bigger impact on outcomes, but also noted that the treatment regimen used in older papers may not be clinically relevant today. The overall evidence rating in this table reflects the results for the critical outcomes of dystocia and 'large for gestational age' as defined by the guideline, and also the lack of any real benefit when insulin is added to diet. ᵃ The guideline committee noted that although more women receiving diet required additional insulin than those receiving standard care, when diet plus insulin was compared to diet alone, there were no significant differences for treatment failure between treatment groups. ᵇ The guideline committee reported that meta-analysis was not possible for most studies due to variation in diagnostic criteria and the types of interventions used. Therefore, sometimes a range of GRADE is reported for an outcome. ᶜ No events in either group. ᵈ Treatment failure was reported by two studies. One found no significant difference between groups, the other favoured the intervention (exercise).
This evidence table is related to the following section/s:
This table is a summary of the analysis reported in a guideline (underpinned by a systematic review) that focuses on the above important clinical question.
Confidence in the evidence is moderate or low to moderate where GRADE has been performed and there may be no difference in effectiveness between the intervention and comparison for key outcomes.
Population: Pregnant women with gestational diabetes, but who are presumed to not have pre-existing diabetes
Intervention: Metformin
Comparison: Insulin
| Outcome | Effectiveness (BMJ rating)? | Confidence in evidence (GRADE)? |
|---|---|---|
Spontaneous vaginal birth | See note ᵃ | Low |
Induction of labour | See note ᵇ | Low to Moderate ᵇ |
Vacuum extraction | No statistically significant difference | Low |
Caesarean section ᶜ | No statistically significant difference | Moderate |
Assisted vaginal delivery | No statistically significant difference | Very Low |
Treatment failure (need for additional insulin) | See note ᵈ | Moderate |
Acceptability | See note ᵉ | Moderate |
Large for gestational age | No statistically significant difference | Very Low |
Shoulder dystocia | No statistically significant difference | Very Low |
Recommendations as stated in the source guideline The National Institute of Health and Care Excellence (NICE) guideline on Diabetes in Pregnancy makes the following recommendations: Offer metformin to women with gestational diabetes if blood glucose targets are not met using changes in diet and exercise within 1–2 weeks. Offer insulin instead of metformin to women with gestational diabetes if metformin is contraindicated or unacceptable to the woman.
Note The guideline committee stated that the critical outcomes were shoulder dystocia and large for gestational age. Mode of delivery and treatment failure were also considered important for decision making. See the full guideline for any additional outcomes reported by the included studies. ᵃ One small trial (n=30) found fewer spontaneous vaginal births among women who received metformin compared with those who received insulin, while the other trial (n=97) found no difference. ᵇ A meta-analysis of 3 studies (n=343) found a reduced risk of induction of labour in women who received metformin (low quality of evidence as assessed by GRADE). However, a further large study (n=733) found no significant difference (GRADE moderate). ᶜ The guideline also reports results for elective and emergency caesarean section, both of which also showed no significant difference between groups (GRADE low). ᵈ This outcome was only reported for the group receiving metformin. 36% (206/568, 5 randomised controlled trials) of women who received metformin had a treatment failure requiring insulin. ᵉ The guideline reported acceptability measured by 3 questions: ‘How often did you forget to take your medication?’; ‘Which medicine would you choose in another pregnancy?’; and ‘Which part of your diabetes treatment was the easiest?’. Although more women forgot to take metformin compared with insulin, there was a significant difference in favour of metformin for subsequent pregnancies. Women given metformin were also more likely to say that taking the medication was the easiest part of their treatment.
This evidence table is related to the following section/s:
Cochrane Clinical Answers
Cochrane Clinical Answers (CCAs) provide a readable, digestible, clinically focused entry point to rigorous research from Cochrane systematic reviews. They are designed to be actionable and to inform decision making at the point of care and have been added to relevant sections of the main Best Practice text.
- Can diet and exercise interventions help to prevent gestational diabetes mellitus?
- Can dietary advice interventions during pregnancy prevent gestational diabetes mellitus and associated adverse health outcomes among mothers?
- Which interventions prevent pregnant women from developing gestational diabetes?
- For women with gestational diabetes, what are the effects of lifestyle, dietary, and exercise interventions?
- How do different oral anti‐diabetic pharmacological therapies compare for treatment of women with gestational diabetes?
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